1. Field of the Invention
The present invention relates to humidity and/or temperature control devices, and more particularly relates to a humidity or temperature control device having low power requirements and improved reliability.
2. Description of the Prior Art
Humidity sensors and humidity detection devices are well known in the art. Humidity control devices are commonly used in microwave ovens, humidifiers, dehumidifiers and clothes dryers, and in various electronic devices to minimize condensation due to sudden changes in temperature.
As discussed in U.S. Pat. No. 4,481,813 to Tanei, et al., known methods of humidity detection include optical changes in light reflection or water absorption spectra, changes in resonant frequency of piezo-resonators, changes in capacitance or changes in electric resistance. However, the method utilizing optics requires a highly precise optical system which is relatively expensive for use in domestic appliances. The methods of humidity detection using piezo-resonators or detecting changes in capacitance are also not preferred methods since the associated electronic circuits tend to be fairly complex. Accordingly, the method of humidity detection based upon changes in resistance has become the most preferred due to the simplicity of the associated electronic circuit.
Generally, the control circuits that employ a resistive humidity sensor may be categorized as either an alternating current (AC) type or a direct current (DC) type, depending upon the power source for the humidity detection circuit. The humidity control circuits respond to changes in the resistance of the humidity sensor caused by changes in humidity. Most commonly used humidity sensors exhibit a negative coefficient response whereby resistance of the humidity sensor decreases with an increase of moisture absorbed into the sensor surface. However, sensors have also been developed which exhibit an increase in electrical resistance with an increase in humidity detected by the sensor, i.e., a positive coefficient.
In order to avoid the more complicated circuits associated with AC driven sensors, humidity sensors driven by direct current (DC) have been developed. For example, U.S. Pat. No. 4,481,813 to Tanei, et al. discloses a direct current (DC) type humidity sensor whose resistance decreases with an increase in humidity. The humidity sensor comprises a pair of counterposed electrodes, a humidity-sensitive layer of insulating porous metal oxide provided on and between the counterposed electrodes, and an organic polymer coating layer provided on the humidity-sensitive layer.
Yet another humidity sensor is disclosed in U.S. Pat. No. 4,938,928 to Koda, et al. This humidity sensor comprises a metal heat generating member formed with a heat-resistant insulating coating and an atmosphere-sensitive layer supported on the coating. At least one electrode is connected to the atmosphere-sensitive layer for detecting a combustible gas, humidity or the like.
The most common circuit for measuring and controlling humidity utilizes an integrated circuit which outputs a signal to a sensor. A return signal is responsive to a resistance or capacitance of the sensor circuit and this return signal is analyzed by a microprocessor to determine the relative humidity. Such a circuit is described in U.S. Pat. No. 5,200,589 to Kim.
The Kim patent discloses a microwave oven having a fan motor whose rotation is controlled based upon the relative humidity within the oven. A control circuit is utilized which comprises a microprocessor, a zero balance circuit, an absolute humidity sensor, a humidity detecting circuit and a relay driving circuit. More specifically, the control circuit compensates for the humidity that exists before the heating cycle in the microwave oven by zero balancing the humidity detecting circuit during the beginning stage of the cycle. The Kim patent also discloses using a zero crossing circuit coupled to the microprocessor to help phase control the fan motor of the oven. This is accomplished by triggering a phototriac at various phase angles to thus control the speed of the fan to thereby control the humidity of the oven cavity by controlling the exhaust.
Many conventional humidity sensors and circuits have several common disadvantages. One such disadvantage is that the power requirements to drive the circuits tend to be relatively high. Some conventional power control circuits employ a zero-crossing detector directly driving a phototriac. Such circuits may draw 30 milliamperes or more of current. Additionally, variations in line voltage may cause conventional humidity detection circuits to be inaccurate. It is also known that temperature changes may affect the resistance of the humidity sensor used in conventional circuits.
Conventional temperature control circuits also have a number of disadvantages. They are often required to switch on and off high power devices, such as heaters, which may consume 750 to 1500 watts of power at 110 volts AC. A heater may draw high currents over relative long stretches of house wiring every time it is energized by the temperature control circuit. The result is large voltage drops across the house wiring to which the heater is connected. Other electrical devices, such as lamps or lighting fixtures, connected to the same wiring as the heater, may be affected.
For example, when the heater is turned on by the temperature control circuit, lights may dim significantly. This continual dimming and brightening of lights every time the heater is cycled on and off is noticeable and disconcerting to occupants in the dwelling where the heater is being used. It is the abrupt transition from bright to dim to bright again that is so noticeable.